The existing long term evolution (LTE) cellular network is operated only in a licensed band. Notwithstanding that a technology for continuously increasing capacity has been developed, as a demand for high-capacity and fast data service is increased, an LTE standard is not limited to the existing licensed band and adopts a method for accommodating an unlicensed band having a rich frequency bandwidth to increase capacity. The capacity increasing method is now in a standardization stage of 3rd generation partnership project (3GPP).
However, unlike a licensed band not hindered by other providers or other devices but having a high degree of freedom, for an unlicensed band, a coexistence problem with devices operated in another unlicensed band needs to be solved. That is, a channel approach and occupation method for temporarily using, by a device, a corresponding channel at every opportunity without greatly lowering performance of other devices on the same unlicensed channel are required.
To solve the coexistence problem, a method known as ‘transmission method after sensing of carrier’ (for example, clear channel assessment (CCA) or listen before talk (LBT)) has been widely used. The channel approach method is first performed by channel monitoring. That is, a device senses activity of an unlicensed band channel shared with other devices and if energy of the corresponding channel is measured, puts off a wireless signal transmission but if energy of the corresponding channel is not sensed (that is, channel idle status), uses the corresponding channel (transmits or outputs a wireless signal).
If the device senses the channel idle status and then transmits the signal, other devices determine that the energy of the corresponding channel is sensed and thus the corresponding channel is busy and put off the signal transmission. That is, the channel approach method of the unlicensed band may be one of a kind of time division multiple access methods for dividing time to make a plurality of devices approach a radio channel.
Further, an LTE frame of an unlicensed band needs to be time-synchronized with an LTE frame operated in a licensed band. A technology for improving signal transmission efficiency under the situation that the restrictions are present is required.